OpenCV: change resolution of VideoCapture while it is capturing - c++

Question

I am using OpenCV 3.1.0 on Windows 10 64-bit. I would like to be able to set the resolution of webcam while webcam still working. It's easy to set resolution after camera working. But I can't set resolution when webcam is capturing.
Here is my code:
cv::VideoCapture cap(0);
cap.set(cv::CAP_PROP_FRAME_WIDTH, 0x7FFFFFFF); // working
cap.set(cv::CAP_PROP_FRAME_HEIGHT, 0x7FFFFFFF); // working
while (true) {
cv::Mat frame;
cap >> frame;
if (!frame.data) continue;
cv::imshow("test", frame);
if (cv::waitKey(1) >= 0) break;
int newHeight = 500 + rand() % 4 * 100;
cap.set(cv::CAP_PROP_FRAME_HEIGHT, newHeight); // not working
}
int newHeight = 500 + rand() % 4 * 100;
cap.set(cv::CAP_PROP_FRAME_HEIGHT, newHeight); // not working

Answer 1

My best guess is the values for CAP_PROP_FRAME_HEIGHT you are attempting are not supported by the webcam. If you hook your camera up to a Linux box, you can use v4l2-ctl -d 0 --list-formats-ext to list the supported video formats. Here an excerpt of the output for a Microsoft LifeCam Cinema:
Index : 1
Type : Video Capture
Pixel Format: 'MJPG' (compressed)
Name : Motion-JPEG
Size: Discrete 640x480
Interval: Discrete 0.033s (30.000 fps)
Interval: Discrete 0.050s (20.000 fps)
Interval: Discrete 0.067s (15.000 fps)
Interval: Discrete 0.100s (10.000 fps)
Interval: Discrete 0.133s (7.500 fps)
Size: Discrete 1280x720
Interval: Discrete 0.033s (30.000 fps)
Interval: Discrete 0.050s (20.000 fps)
Interval: Discrete 0.067s (15.000 fps)
Interval: Discrete 0.100s (10.000 fps)
Interval: Discrete 0.133s (7.500 fps)
...
I have not checked recently whether on Windows there is something similar to v4l2-ctl, which uses UVC to query the info from the camera. UVC is typically supported by recent webcams.

Answer 2

The problem is I only set a random height, and webcam only supports its preset resolution. So it select a best matched preset resolution to show it.